Computer systems use memory devices, such as dynamic random access memory (“DRAM”) devices, to store data that are accessed by a processor. These memory devices are normally used as system memory in a computer system. In a typical computer system, the processor communicates with the system memory through a processor bus and a memory controller. The processor issues a memory request, which includes a memory command, such as a read command, and an address designating the location from which data or instructions are to be read. The memory controller uses the command and address to generate appropriate command signals as well as row and column addresses, which are applied to the system memory. In response to the commands and addresses, data are transferred between the system memory and the processor.
Although the operating speed of memory devices has continuously increased, this increase in operating speed has not kept pace with increases in the operating speed of processors. Even slower has been the increase in operating speed of memory controllers coupling processors to memory devices. The relatively slow speed of memory controllers and memory devices limits the data bandwidth between the processor and the memory devices.
In addition to the limited bandwidth between processors and memory devices, the performance of computer systems is also limited by latency problems that increase the time required to read data from system memory devices. More specifically, when a memory device read command is coupled to a system memory device, such as a synchronous DRAM (“SDRAM”) device, the read data are output from the SDRAM device only after a delay of several clock periods. Therefore, although SDRAM devices can synchronously output burst data at a high data rate, the delay in initially providing the data can significantly slow the operating speed of a computer system using such SDRAM devices.
One approach to alleviating the memory latency problem is to use multiple memory devices coupled to the processor through a memory hub. In a memory hub architecture, a system controller or memory controller is coupled over a high speed link to several memory modules. Typically, the memory modules are coupled in a point-to-point or daisy chain architecture such that the memory modules are connected one to another in series. Thus, the memory controller is coupled to a first memory module over a first high speed link, with the first memory module connected to a second memory module through a second high speed link, and the second memory module coupled to a third memory module through a third high speed link, and so on in a daisy chain fashion.
Each memory module includes a memory hub that is coupled to the corresponding high speed links and a number of memory devices on the module, with the memory hubs efficiently routing memory requests and memory responses between the controller and the memory devices over the high speed links. Computer systems employing this architecture can have a higher bandwidth because a processor can access one memory device while another memory device is responding to a prior memory access. For example, the processor can output write data to one of the memory devices in the system while another memory device in the system is preparing to provide read data to the processor. Moreover, this architecture also provides for easy expansion of the system memory without concern for degradation in signal quality as more memory modules are added, such as occurs in conventional multi drop bus architectures.
FIG. 1 is a block diagram of a system memory 102 that includes memory modules 104a and 104b. The memory module 104a is coupled to a system controller 108 through a downstream link 128 and an upstream link 136. Each of the memory modules 104a, 104b includes a memory hub 112, which includes a link interface 116. In the memory module 104a, the link interface 116 is connected to the system controller 108 by the links 128, 136. The link interface 116 includes a downstream reception port 124 that receives downstream memory requests from the system controller 108 over the downstream link 128, and includes an upstream transmission port 132 that provides upstream memory responses to the system controller over the upstream link 136
The system controller 108 includes a downstream transmission port 140 coupled to the downstream link 128 to provide memory requests to the memory module 104a, and also includes an upstream reception port 144 coupled to the upstream link 136 to receive memory responses from the memory module 104a. The ports 124, 132, 140, 144 and other ports to be discussed below are designated “physical” interfaces or ports since these ports are in what is commonly termed the “physical layer” of a communications system. In this case, the physical layer corresponds to components providing the actual physical connection and communications between the system controller 108 and system memory 102 as will be understood by those skilled in the art.
The nature of the reception ports 124, 144 and transmission ports 132, 140 will depend upon the characteristics of the links 128, 136. For example, in the event the links 128, 136 are implemented using optical communications paths, the reception ports 124, 144 will convert optical signals received through the optical communications path into electrical signals and the transmission ports 140, 132 will convert electrical signals into optical signals that are then transmitted over the corresponding optical communications path.
In operation, the reception port 124 captures the downstream memory requests and provides the captured memory request to local hub circuitry 148, which includes control logic for processing the request and accessing the memory devices 156 over a bus system 152 to provide the corresponding data when the request packet is directed to the memory module 104a. The reception port 124 also provides the captured downstream memory request to a downstream transmission port 160 on a bypass bus 180. The downstream transmission port 160, in turn, provides the memory request over the corresponding downstream link 128 to a downstream reception port 124 in the adjacent downstream memory module 104b. The port 124 in module 104b operates in the same way as the corresponding port in the module 104a, namely to capture the memory request and provide the request to the local hub circuitry 148 for processing and to provide the request to a downstream transmission port 160. The port 160 in the module 104b then operates in the same way as the corresponding port in module 104a to provide the memory request over the corresponding downstream link 128 to the next downstream memory module (not shown in FIG. 1).
The memory hub 112 in the module 104a further includes an upstream reception port 164 that receives memory responses over the corresponding upstream link 136 from an upstream transmission port 132 in the adjacent module 104b. An upstream transmission port 132, in turn, provides the response over the upstream link 136 to the upstream physical reception port 144 in the system controller 108. Each of the memory modules 112 includes a corresponding downstream reception port 124, upstream transmission port 132, downstream transmission port 160, and upstream reception port 164. Moreover, these ports 124, 132, 160, 164 in each module 104b operate in the same way as just described for the corresponding ports in the module 104a. 
In addition to the memory responses from the downstream hubs, the local hub circuitry 148 also receives memory responses from a local memory 156. The local memory 156 may be a DRAM type memory device or other suitable memory devices as will be appreciated by those skilled in the art. The local hub circuitry 148 provides the memory responses from the local memory 156 to the upstream transmission port 132 for transmission over the upstream link 136 to the upstream reception port 144 of the controller 108. Thus, the local hub circuitry 148 must monitor and control transmission of memory responses to the system controller 108 from the downstream memory module 104b and from the local memory 156. Since the hub circuitry 148 must monitor and control transmission of memory responses to the system controller 108 from the downstream memory module 104b and the local memory 156, the hub circuitry 148 must determine the priority of transmission of the memory responses. The hub circuitry 148 also must efficiently switch the transmission of memory responses from one source to another source. The hub circuitry 148 also must switch transmission of memory responses from one source to another source at an appropriate time.
The system controller 108 can control the timing of the memory responses inside the memory hubs 112. However, if there are a large number of memory hubs 112 coupled to the system controller 108, it becomes complicated for the system controller 108 to efficiently determine the priority of transmission of memory responses and to do the scheduling in all the memory hubs 112. Also when the system controller 108 controls the scheduling of memory responses inside the memory hubs 112, the bandwidth available for data transmission is reduced.
Accordingly, there is a need for a system and method for efficiently determining the priority of transmission of the memory responses inside the memory hub 112. There is a need for a system and method for efficiently switching transmission of the memory responses from one source to another source inside the memory hub 112. There is a need for a system and method for efficiently switching transmission of the memory responses from one source to another source at an appropriate point.